Of the five total progeny classes, three are treated as nominally “homozygous” (CC011/CC011) in a standard backcross analysis, but in fact, heterozygosity for the CAST/EiJ and 129S1/SvImJ alleles at Ccc4 is associated with higher colitis scores, while neither allele has detectable effect in homozygosity (Fig. 7). This evidence concerns the gene CAST and colitis.